Reprecipitation Processes Research Articles

The ordering behaviour of reedmergnerite, NaBSi3O8

Reedmergnerite (NaBSi3O8) has been synthesised hydrothermally from gels containing 10 wt.% excess SiO2. The degree of B, Si order increases with time at constant temperature and pressure. Complete order is achieved in 250 h at 400° C, Pfluid=1 kbar and in 700 h at 300° C, 1 kbar. Lower pressures and/ or low water contents greatly reduce the rate of ordering. The ordering behaviour of reedmergnerite is insensitive to the composition of the co-existing fluid and this contrasts with the behaviour of albite. It is suggested that ordering takes place by a process of solution and re-precipitation.

The effect of cobalt content on the microstructure of liquid-phase sintered TaCCo alloys

A study has been made of the effect of the cobalt binder-phase content on the microstructure and certain mechanical properties of TaCCo alloys prepared by liquid-phase sintering. The investigation was limited to a composition range of 0 to about 65 vol.% cobalt within which the carbide grains formed a continuous network and beyond which the carbide became discontinuous and segregation of the phases occurred. The relationship between the volume fraction and the degree of carbide grain contact within this range was consistent with the existence of an equilibrium degree of contact determined by the ratio of the carbide grain-boundary energy to the carbide/binder interfacial energy. The rate of the carbide grain growth that occurs during sintering decreases slightly in the range from 7 to 60 vol.% binder phase. It is concluded that, in this range, the growth is controlled by a process of dissolution and reprecipitation at carbide/liquid interfaces. The hardnesses of alloys in the investigated composition range agree well with a semi-empirical equation that predicts that the hardness is dependent on the degree of carbide grain contact and on the mean free path through the binder phase as well as on the volume fraction of the phases. Results indicate that the resistance of the microstructure to cracking is also dependent on these parameters.

Studies on the Substructure of Real Crystals by Optical and Magnetic Resonance Methods

Direct ultramicroscopic studies and measurements of the optical scattering spectrum of ``pure'' and doped alkali halide crystals are described showing the existence of elementary scattering centers and of assemblies of such centers. The elementary scatterers are established to be impurity precipitates of dimensions not exceeding a few hundred angstroms. The assemblies lead to characteristic ultra-microscopically visible diffraction images representing decorations of branch points of a semi-organized network of dislocations which reflect the state of strain and deformation in the subgrains of the mosaic structure of the single crystals. With increasing temperature the number of elementary scatterers decreases as impurities enter the host lattice into true solid solution, and the size of the assemblies diminishes. Electron spin resonance experiments on doped alkali halides are discussed which provide direct information on the process of dissolution and reprecipitation of impurity ions. Results on strong exchange effects in the electron spin resonance of the systems MnCl2–NaCl and MnCl2–KCl are presented which are explained in terms of the substructure of very small impurity precipitates deduced from the optical investigations.

New forms of instruments for showing the presence and amount of combustible gas in the air

Mineralized tissues such as teeth and bones consist primarily of highly organized apatitic calcium-phosphate crystallites within a complex organic matrix. The dimensions and organization of these apatite crystallites at the nanoscale level determine in part the physical properties of mineralized tissues. After death, geological processes such as diagenesis and dolomitization can alter the crystallographic properties of mineralized tissues through cycles of dissolution and re-precipitation occurring in highly saline environments. Inspired by these natural exchange phenomena, we investigated the effect of hypersalinity on tooth enamel. We discovered that magnesium ions reacted with human tooth enamel through a process of dissolution and re-precipitation, reducing enamel crystal size at the surface of the tooth. This change in crystallographic structure made the teeth harder and whiter. Salt-water rinses have been used for centuries to ameliorate oral infections; however, our discovery suggests that this ancient practice could have additional unexpected benefits.Here we describe an approach inspired by natural geological processes to modify the properties of a biomineral – human tooth enamel. In this study we showed that treatment of human tooth enamel with solutions saturated with magnesium induced changes in the nanocrystals at the outer surface of the protective enamel layer. As a consequence, the physical properties of the tooth were modified; tooth microhardness increased and the color shade became whiter, thus suggesting that this method could be used as a clinical treatment to improve dental mechanical properties and esthetics. Such an approach is simple and straightforward, and could also be used to develop new strategies to synthesize and modify biominerals for biomedical and industrial applications.